This invention relates to a pneumatic radial tire and more particularly to a heavy duty radial tire for trucks or buses, whose durability is improved by preventing separations occurring at ends of a belt for reinforcing a tread and at ends of carcass plies for principally reinforcing the tire.
In order to improve the durability of bead portions of pneumatic radial tires, techniques of turn up of carcass plies and materials and constructions of chafers or stiffeners as reinforcements have been generally investigated. However, with all proposed solutions, applications are limited to tires of some particular sizes although they are relatively effective for tires of some limited sizes. Moreover, proposed solutions sometimes tend to increase costs of tires. Therefore, a fundamental solution has not been provided yet.
In order to obtain required performances of tires, in general, carcass configurations have been determined so as to expand uniformly when filled with inner pressure. The equilibrium configuration curve of a carcass line of a tire has been used in this industrial field, which is based on the so-called "natural equilibrium theory" assuming that carcass cords are not extensible and deformations of the tire are not affected by inner pressures. A carcasses of such a tire is uniformly subjected to tensile forces and therefore its various performances are good. However, such a tire is a theoretical one which could not exist in fact. It is considered that an actual tire which approximates to such a theoretical tire as much as possible can be obtained by making the actual tire which uniformly inflates or is similar in figure before and after the inflation. An actual tire obtained in this manner has a "natural equilibrium shape" of a carcass. In the following publications disclosed hitherto measures for controlling the change in shape of tires when they are filled with inner pressure have been employed.
In U.S. Pat. No. 4,155,392, when a tire is being filled with inner pressure, maximum width portions of the tire are displaced radially inwardly to reduce tensile strains occurring in sidewalls, thereby improving the durability of the life of the tire referring to FIG. 3 of the U.S. Pat. No. 4,155,392. In this case, however, parts of a tread and shoulders are also moved axially and radially inwardly to reduce initial tensions acting upon a belt of the tire so that moving performances of the tire and durability at ends of the belt are detrimentally affected.
U.S. Pat. No. 4,481,994 discloses a technique similar to the measure disclosed in the above United States Patent specification. In this case, portions extending from ends of a belt to shoulders are also moved axially and radially inwardly of a tire when filling normal pressure. As a result, initial tensions of the belt become small which detrimentally affect various performances of the tire.
Moreover, U.S. Pat. No. 4,513,802 discloses a feature of changing a tire configuration to reduce the rolling resistance of a tire. In this case, however, as radially outward portions of sidewalls outwardly expand when filling normal inner pressure, expanding deformations are insufficient in bead portions and tread portion. Therefore, strain distributions in the tire resulting from the expanding deformations are improper and therefore sufficient durability for heavy duty tires is not obtained.